As is well known in the art, in electrophotographic developing methods, in general, a photosensitive member formed of a photoconductive material such as selenium, OPC (organic semiconductor), α-Si or the like has been used to form an electrostatic latent image thereon by various means. Then, by using a magnetic brush method or the like, a toner having a polarity reverse to that of the latent image is attached thereonto by the electrostatic force to develop the latent image.
In the above developing step, there is used a two-component system developer comprising a toner and a carrier. The carrying particles called a magnetic carrier acts for imparting an appropriate positive or negative electric charge amount to the toner by frictional electrification, and delivering the toner into a developing zone near the surface of the photosensitive member on which the latent image is formed, by a developing sleeve in which magnets are accommodated, using a magnetic force thereof.
In recent years, copying machines or printers of the above electrophotographic developing type have been rapidly digitalized and formed into a composite structure, so that there is a further increasing demand for those apparatuses having a high performance, a high image quality and a high copying or printing speed. With the market's requirements for personalization and space saving, etc., reduction in size of the electrophotographic image-forming apparatuses has been promoted. In particular, it has been demanded to enhance an image quality of full-color images up to a higher level close to a high-grade printing or a silver salt photo. For this reason, in order to faithfully visualize fine latent images for a long period of time, it is important to keep stable electrification of the developer. Thus, in order to stably maintain these properties of the developer, it is required that a carrier used in the developer has a long service life such that various properties of the carrier having the electrification performance such as a charging property and an electric resistance can be stably maintained over a long period of time.
Hitherto, there tends to arise such a problem that the carrier constituting the developer is allowed to remain within a developing device and subjected to repeated frictional contact with a toner, and therefore suffers from significant change in surface conditions thereof with time, resulting in undesirable change in quality of the obtained images. Such a problem tends to be mainly caused by two phenomena, namely, a phenomenon that the toner is firmly adhered onto the surface of the respective carrier particles, so that the charging property inherent to the carrier is lost (i.e., a so-called spent toner), and such a phenomenon that a resin coating layer formed on the surface of the respective carrier particles is peeled off owing to friction with the toner, so that leak sites are formed thereon, thereby causing significant change in electric resistance thereof.
In view of these problems, in order to prevent the carrier from suffering from occurrence of spent toner, there has been conventionally proposed the method in which the surface of the carrier is coated with various resins. For example, it is known that the surface of the respective carrier core particles is coated with a releasable resin such as a fluororesin and a silicone resin. Such a coated carrier not only can be imparted with various functions such as a good charging property and a well-controlled electric resistance, but also hardly suffers from occurrence of spent toner upon the development because the surface thereof is coated with the low-surface energy substance. As a result, the carrier has a stable charge amount, and the developer using the carrier exhibits a long service life.
On the other hand, the carrier has been required to have a certain suitable electric resistance ranging from about 1×108 to 1×1016 Ω·cm. More specifically, when the carrier has an electric resistance value as low as 1×106 Ω·cm like the carrier comprising iron particles, there tend to arise the problems such as attachment of the carrier to image-bearing portions of a photosensitive member owing to injection of electric charges from a sleeve, and occurrence of defective latent images or lack of obtained images owing to escape of latent image-forming electric charges through the carrier. Also, when a thickness of the insulating resin layer is increased, the electric resistance value thereof tends to become too high, so that it may be difficult to leak electric charges of the carrier, and the toner has an excessively high charge amount. As a result, although images having a sharp edge are obtained, there tends to arise such a problem that an image screen having a large displaying area has a considerably low image density at a central portion thereof.
Further, when the electric resistance value of the carrier has a large voltage dependency, the obtained image tends to generally has no gradation, so that even when using the carrier for a developer in copying machines or printers, it may be difficult to obtain images having a high image quality, and the applications thereof tend to be limited.
In general, as the carrier constituting a two-component system developer, there are well known an iron powder carrier, a ferrite carrier, a carrier of a binder type in which magnetic particles are dispersed in a binder resin, and a carrier of a coated type in which a magnetic material is coated with a resin.
The iron powder carrier and ferrite carrier are usually used in the form of resin-coated particles. However, since the iron powder carrier has a true specific gravity as large as 7 to 8 g/cm3 whereas the ferrite carrier has a true specific gravity as large as 4.5 to 5.5 g/cm3, a large driving force is required for stirring these carriers in the developing device, resulting in significant mechanical damage to the device, occurrence of spent toner as well as deterioration in charging property of the carrier itself and facilitated damage to the photosensitive member. Further, since the adhesion between the surface of the iron powder carrier or ferrite carrier and the coating resin is not good, the coating resin tends to be gradually peeled off during use with the time, thereby causing variation in the charging property. As a result, the problems such as formation of defective images and adhesion of the carrier tend to be caused.
The carrier of a magnetic material-dispersed type comprising spherical magnetic composite particles formed from magnetic iron oxide particles and a phenol resin as described in Japanese Patent Application Laid-Open (KOKAI) No. 2-220068 is far excellent in adhesion to coating resins as compared to the iron powder carrier or ferrite carrier and, therefore, suffers from substantially no problem concerning peeling-off of the coating resins therefrom during use.
With the recent progress of coloration, there is an increasing demand for carriers which can be enhanced in various properties for obtaining high-quality images and has a long service life capable of stably maintaining the various properties over a long period of time. However, when using the above carriers, there tends to arise such a problem that the coating resins are abraded owing to impingement between particles, mechanical agitation of the particles within a developing device and thermal stress thereon, or an extent of prevention of peeling-off of the coating resins from the carrier tends to be insufficient. In addition, since the spherical magnetic composite particles constituting a core material of the magnetic material-dispersed type carrier have a low electric resistance value, when the resin coating layer on the carrier suffers from peeling-off, there tend to arise the problems such as leak of electric charges upon the development and poor gradation owing to a large voltage dependency of the electric resistance value.
In particular, in recent years, with the shifting to maintenance-free systems, a developer used therein is often required to have a durability over a long period of time which may correspond to almost a whole service life of the maintenance-free machines. Therefore, it is strongly required to provide the magnetic carrier which is free from peeling-off of the coating resin owing to abrasion and occurrence of spent toner, and exhibits a sufficient electric resistance with a low voltage dependency.
Conventionally, there are known the methods directed to surface conditions of a magnetic carrier in which irregularities are formed on a surface of the respective carrier particles to control a surface configuration thereof.
For example, there are known the technique of controlling surface conditions of resin-dispersed type carrier particles or spray-dried carrier particles according to a ten-point mean roughness (Rz) and a standard deviation of roughness thereof (Patent Document 1); the techniques of forming irregularities on a surface of respective carrier particles by coating the particles with a resin comprising a protrusion-forming material to control a surface configuration of the respective particles according to a ten-point mean roughness (Rz) or according to a difference in height between the irregularities and the number of protrusions present thereon (Patent Documents 2 and 3); the technique of controlling surface conditions of carrier particles according to an arithmetic mean roughness Ra and a mean spacing of profile irregularities Sm thereof by varying calcining conditions (Patent Document 4); the technique of controlling surface conditions of carrier particles according to an arithmetic mean roughness Ra and a depth of a groove between the adjacent protrusions by varying calcining conditions to form stripe-patterned protruded portions on the surface of the respective particles (Patent Document 5); the technique of subjecting carrier particles to acid treatment to form a honeycomb-shaped surface thereon, and controlling a BET specific surface area of the particles so as to satisfy a calculation formula: S=a×Db (wherein S: BET specific surface area (m2/g) of core particles; D: average particle diameter (μm) of the core particles; a: coefficient, 3≦a≦22; b: coefficient, b=−1.05) (Patent Document 6); the technique of forming fine irregularities owing to plate-shaped metal oxide particles on a surface of respective particles of a magnetic carrier to control surface conditions of the magnetic carrier according to a fluidity rate thereof (Patent Document 7); etc.
Further, there is known the method in which composite particles comprising ferromagnetic iron oxide fine particles and a cured phenol resin are coated with a melamine resin. For example, there are known the technique of coating a surface of respective composite core particles comprising ferromagnetic fine particles and a cured phenol resin with a melamine resin to impart a high electric resistance thereto (Patent Document 8); the technique of forming a coating layer comprising a copolymer resin obtained by curing at least one resin selected from a melamine resin, an aniline resin and a urea resin with a phenol resin, on a surface of respective composite core particles comprising iron oxide particles and a cured phenol resin to control an electric resistance value of a carrier (Patent Document 9); etc.